As is well known; microwave power amplifiers are non-linear devices whose amplitude and phase transfer function varies depending on a number of factors including input signal level, frequency, temperature, power supply fluctuations and age. Power amplifiers for wireless communications apparatus typically have to operate throughout a large part of their dynamic range, some of which is highly non-linear. These applications therefore mean that the amplifiers are especially prone to some of these factors. This results in a distorted signal output giving rise to the generation of spurious emissions in the adjacent and nearby frequency channels.
The non-linear effects of power amplification on a spread-spectrum modulated signal are particularly pronounced and introduce sidebands, termed ‘regrowth sidebands’ which are characteristic of amplifier distortion. Regrowth sidebands are a system problem since they can potentially interfere with neighbouring communication channels. Specification limits on regrowth sidebands are therefore stringently specified in most cellular communication standards, including those for the new third generation mobile systems such as UMTS.
Feed-forward linearisation circuitry is typically employed in cellular power amplifiers to adjust the output of the amplifier to compensate for its non-linear characteristics. (Other linearising methods include direct RF feedback and envelope feedback.) A recent advance in this field has been the use of predistortion circuitry which adjusts the much smaller input signal to the amplifier to compensate “in advance” for expected non-linearity in the amplifier. Predistortion amplifiers are less complicated than feedforward amplifiers which require the modification of the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis. Feed forward amplifiers also require a separate error amplifier handling similar power levels to the main amplifier which significantly increases the system cost and power consumption. A predistortion arrangement is described in European Patent Publication EP 1 011 192, which corresponds to applicants issued U.S. Pat. No. 6,275,685 issued Aug. 14, 2001 (Wessel). Other predistortion arrangements are described in U.S. Pat. No. 4,700,151 (Nagata) and U.S. Pat. No. 5,049,832 (Cavers).
The predistorter is a signal processing element inserted before the (nonlinear) High Power Amplifier which modifies the input signal with a nonlinearity complimentary to that of the amplifier. On passing through the High Power Amplifier the predistortion sidebands on the modified signal and the distortion products from the amplifier cancel each other, giving a greatly improved output spectrum. This has an advantage over the traditional feedforward linearization architecture in that the signal processing is all carried out on a small signal at the amplifier input, resulting in great savings in cost and power consumption.
However, to achieve more than a few dB reduction in adjacent channel regrowth, the predistorter typically needs to be made adaptive. In this way the predistorter can be continuously adjusted to maintain high performance even as the amplifier response varies with temperature, operating frequency, power supply and aging.
One of the crucial parts of an adaptive predistorter system is a system which estimates the error between the ideal (input) signal and the corrupted system output in order to drive the adaption process. Depending on the characteristics of the signal, estimating this error to the required degree of accuracy is extremely difficult. The performance of the error detector system can in fact be limiting on the level of regrowth suppression achievable.
Wessel also describes such an error detector system, however even higher levels of accuracy are desirable, also Wessel requires the detectors concerned to maintain closely matched performance over power and temperature which may have cost implications. It may also require factory adjustment to optimize spurious performance.